The paper industry continuously strives to improve paper quality, increase process speeds, and reduce manufacturing costs. Manufacture of paper or paperboard involves producing an aqueous slurry of cellulosic wood fiber, which may also contain inorganic mineral extenders or pigments. The slurry is deposited on a moving wire or fabric whereupon the paper sheet is formed from the solid components by draining the water. This process is typically followed by pressing and drying sections. A variety of organic and inorganic chemicals are often added to the slurry before the sheet forming process to decrease costs, increase efficiency, and/or impart specific properties to the final paper product.
Typically, the limiting step in achieving faster process speeds in paper manufacturing is the dewatering or drainage of the fibrous slurry on the wire. Depending upon machine size and speed, this step removes large volumes of water in a very short period of time. The efficient removal of this water is critical in maintaining process speeds. Chemicals are sometimes added to the pulp before the wire to improve drainage and retention performance. These chemicals and chemical programs are often called retention and/or drainage aids. Retention aids are used to increase retention of fine furnish solids in the web during the turbulent process of draining and forming the paper web. Without adequate retention of these fine solids, they become lost in the process effluent or accumulate to excessively high concentrations in the recirculating white water loop leading to production difficulties. Insufficient retention of these fine solids and the disproportionate quantity of chemical additives which are adsorbed on their surfaces generally reduces paper quality characteristics, such as opacity, strength, and sizing.
Several forms of retention and drainage aids are known. For example, medium molecular weight diallyldimethylammonium chloride/acrylamide copolymers as retention and drainage aids are reviewed in Hunter et al., “TAPPI 99 Preparing for the Next Millennium,” vol. 3, pp. 1345-1352, TAPPI Press (1999). U.S. Pat. No. 6,605,674 B1 discloses free radical polymerization of structurally modified cationic polymers and use of these polymers as retention and drainage aids in papermaking processes. U.S. Pat. No. 6,071,379 discloses the use of diallyl-N,N-disubstituted ammonium halide/acrylamide dispersion polymers as retention and drainage aids in papermaking processes. U.S. Pat. No. 5,254,221 discloses a method of increasing retention and drainage in a papermaking process using a low to medium molecular weight diallyldimethylammonium chloride/acrylamide copolymer in combination with a high molecular weight dialkylaminoalkyl(meth)acrylate quaternary ammonium salt/acrylamide copolymer.
U.S. Pat. No. 6,592,718 B1 discloses a method of improving retention and drainage in a papermaking furnish comprising adding to the furnish a diallyl-N,N-disubstituted ammonium halide/acrylamide copolymer and a high molecular weight structurally-modified, water-soluble cationic polymer. U.S. Pat. Nos. 5,167,776 and 5,274,055 disclose ionic, cross-linked polymeric microbeads having a diameter of less than about 1,000 nm and use of the microbeads in combination with a high molecular weight polymer or polysaccharide in a method of improving retention and drainage of a papermaking furnish.
Nonetheless, an ongoing need to develop new compositions and processes to further improve retention and drainage performance exists, particularly for use on faster and bigger modern papermaking machines currently being put into use. A particular need exists to improve retention and drainage in mechanical grade papermaking furnishes.